1. Field of the Invention
This invention relates to a nitride-based semiconductor substrate that its epi-characteristics can be kept uniform in plane, even when the nitride-based semiconductor is formed with a mixed crystal of three elements or more, to provide a high yield and a high reliability with its device. Also, this invention relates to a method of making the nitride-based semiconductor substrate, and to an epitaxial substrate for a nitride-based semiconductor light emitting device using the nitride-based semiconductor substrate.
2. Description of the Related Art
Nitride-based semiconductor devices, such as gallium nitride (GaN), indium gallium nitride (InGaN) and gallium aluminum nitride (AlGaN) attract attention for a material of blue light emitting diode (LED) or laser diode (LD). Further, since the GaN-based compound semiconductors have a good heat resistance and environment resistance, they have begun to be applied to other electronic devices.
Thus far, in growing GaN which is a typical nitride-based semiconductor, sapphire single crystal substrates have been used.
However, since the sapphire substrates are not lattice-matched with the GaN, a GaN single crystal film cannot be grown directly on the sapphire substrate. Therefore, a method is developed in which a buffer layer (=low-temperature growth buffer layer) of AlN or GaN is grown on the sapphire substrate at low temperature to buffer a strain in lattice, and then GaN is grown thereon (e.g., JP-A-H04-297023).
By virtue of the low-temperature growth buffer layer, the epitaxial growth of GaN single crystal can be realized. However, the above method still has a problem that the grown GaN has a number of defects since the lattice mismatch between the substrate and the GaN crystal is not perfectly eliminated. It is assumed that the defects will bring some failure in fabricating a GaN-based LD and high-brightness LED.
Because of this, it is desired to develop a GaN free-standing substrate not to cause the lattice mismatch between it and the GaN crystal. Since it is difficult to grow a large ingot of GaN from a melt as in the cases of Si or GaAs, various methods such as the HVPE (hydride vapor phase epitaxy), the ultrahigh temperature and pressure method, and the flux method have been tried to make the GaN free-standing substrate. Especially, the development of the GaN free-standing substrate by HVPE is most advanced. The GaN free-standing substrates by HVPE have gradually begun to be commercially available.
A high-quality GaN is reported which has a thermal conductivity as high as about 2 W/cmK (e.g., D. I. Florescu et al., “High spatial resolution thermal conductivity and Raman spectroscopy investigation of hydride vapor phase epitaxy grown n-GaN/sapphire (0001): Doping dependence”, Journal of Applied Physics 88(6) (2000) p 3295). This value is about five times the sapphire (0.42 W/cmK) and is a very high value close to aluminum (2.4 W/cmK). Thus, by using the high-quality GaN, a free-standing substrate with a high heat radiation property can be produced.
In recent years, nitride-based semiconductor substrates, such as AlGaN, with a mixed crystal composition of three elements or more have been researched.
However, when the nitride-based semiconductor is formed with a mixed crystal of three elements or more, the thermal conductivity will be reduced significantly. For example, in case of AlxGa1-xN (x=0.1), its thermal conductivity becomes reduced to about ¼ of GaN. The reduction in thermal conductivity is most significant in x=0.05 to 0.95. If the substrate with the reduced thermal conductivity is used to conduct the epitaxial growth thereon, fluctuation in temperature will occur to generate nonuniform epi-characteristics in the plane of the substrate. As a result, the yield of the device will lower.